A Multiple Input Multiple Output (MIMO) system refers to a system that can enhance transmission and reception efficiency of data by using multiple transmitting antennae and multiple receiving antennae. The MIMO technology includes a Spatial diversity technique and a Spatial multiplexing technique. The Spatial diversity technique may increase transmission reliability through a diversity gain or may widen a cell range. Thus, the Spatial diversity technique is suitable for data transmission with respect to a user equipment moving at a high speed. And, by transmitting data simultaneously, the Spatial multiplexing technique may increase the data transmission rate without increasing the system bandwidth.
In the MIMO system, each transmitting antenna has an independent data channel. The transmitting antenna may signify a virtual antenna or a physical antenna. A reception subject may estimate a channel with respect to each transmitting antenna of a transmission subject, thereby being capable of receiving data transmitted from each transmitting antenna. Channel estimation refers to a process of recovering a received signal by compensating for a distortion in a signal, which is caused by fading. Herein, fading refers to an effect wherein the intensity of a signal is changed abruptly due to a multi path-time delay in a wireless telecommunications system environment. In order to perform channel estimation, a reference signal commonly known by the transmission subject and the reception subject is required. A reference signal may also simply be referred to as an RS (Reference Signal) or may also be referred to as a Pilot depending upon the applied standard.
Among the diverse downlink reference signals, a user equipment (UE)-specific DMRS (Demodulation RS) for data demodulation is defined. When performing multi-user-MIMO (MU-MIMO), the UE-specific DMRS may be used. Each user equipment may use channel information that is received (or acquired) through a precoding-based DMRS, so as to perform MU-MIMO operations without any interference with other user equipments.